Dynamics of Secretory Membrane Trafficking

Intravital microscopy in mammalian multicellular organisms

Imaging subcellular processes in live animals is no longer a dream. The development of Intravital Subcellular Microscopy (ISMic) combined with the astounding repertoire of available mouse models makes it possible to investigate processes such as membrane trafficking in mammalian living tissues under native conditions. This has provided the unique opportunity to answer questions that cannot be otherwise addressed in reductionist model systems and to link cell biology to tissue pathophysiology.

Toolbox: Creating a systematic database of secretory pathway proteins uncovers new cargo for COPI

A third of yeast genes encode for proteins that function in the endomembrane system. However, the precise localization for many of these proteins is still uncertain. Here, we visualized a collection of ~500 N‐terminally, green fluorescent protein (GFP), tagged proteins of the yeast Saccharomyces cerevisiae. By co‐localizing them with 7 known markers of endomembrane compartments we determined the localization for over 200 of them. Using this approach, we create a systematic database of the various secretory compartments and identify several new residents. Focusing in, we now suggest that Lam5 resides in contact sites between the endoplasmic reticulum and the late Golgi. Additionally, analysis of interactions between the COPI coat and co‐localizing proteins from our screen identifies a subset of proteins that are COPI‐cargo. In summary, our approach defines the protein roster within each compartment enabling characterization of the physical and functional organization of the endomembrane system and its components.

Examination of VDR/RXR/DRIP205 Interaction, Intranuclear Localization, and DNA Binding in Ras-Transformed Keratinocytes and Its Implication for Designing Optimal Vitamin D Therapy in Cancer

Retinoid X receptor (RXR) occupies a central position within the nuclear receptor superfamily, serving as an obligatory partner to numerous other nuclear receptors, including vitamin D receptor (VDR). In the current study, we examined whether phosphorylation of RXRα at serine 260 affects VDR/RXR and VDR interacting protein (DRIP) 205 coactivator recruitment, interactions, and binding of the VDR/human RXRα (hRXRα)/DRIP205 complex to chromatin. Serine 260 is a critical amino acid on the hRXRα that is located in close spatial proximity to regions of coactivator and corepressor interactions. Using fluorescence resonance energy transfer and immunofluorescence studies, we showed that the physical interaction between hRXRα and DRIP205 coactivator was impaired in human keratinocytes with the ras oncogene (HPK1Aras) or transfected with the wild-type hRXRα. Furthermore, the nuclear colocalization of VDR/DRIP205, hRXRα/DRIP205, and VDR/hRXRα/DRIP205 complex binding to chromatin is impaired in the HPK1Aras cells when compared with the normal human keratinocytes (HPK1A cells). However, transfection with the nonphosphorylatable hRXRα (S260A) mutant or treatment with the mitogen-activated protein kinase (MAPK) inhibitor UO126 rescued their nuclear localization, interaction, and binding of the complex to chromatin in the HPK1Aras cells. In summary, we have demonstrated, using highly specific intracellular tagging methods in live and fixed cells, important alterations of the vitamin D signaling system in cancer cells in which the ras-raf-MAPK system is activated, suggesting that specific inhibition of this commonly activated pathway could be targeted therapeutically to enhance vitamin D efficacy.

Rab11-family interacting proteins define spatially and temporally distinct regions within the dynamic Rab11a-dependent recycling system

The Rab11-family interacting proteins (Rab11-FIPs) facilitate Rab11-dependent vesicle recycling. We hypothesized that Rab11-FIPs define discrete subdomains and carry out temporally distinct roles within the recycling system. We used live-cell deconvolution microscopy of HeLa cells expressing chimeric fluorescent Rab11-FIPs to examine Rab11-FIP localization, transferrin passage through Rab11-FIP-containing compartments, and overlap among Rab11-FIPs within the recycling system. FIP1A, FIP2, and FIP5 occupy widely distributed mobile tubules and vesicles, whereas FIP1B, FIP1C, and FIP3 localize to perinuclear tubules. Internalized transferrin entered Rab11-FIP-containing compartments within 5 min, reaching maximum colocalization with FIP1B and FIP2 early in the time course, whereas localization with FIP1A, FIP1C, FIP3, and FIP5 was delayed until 10 min or later. Whereas direct interactions with FIP1A were only observed for FIP1B and FIP1C, FIP1A also associated with membranes containing FIP3. Live-cell dual-expression studies of Rab11-FIPs revealed the tubular dynamics of Rab11-FIP-containing compartments and demonstrated a series of selective associations among Rab11-FIPs in real time. These findings suggest that Rab11-FIP1 proteins participate in spatially and temporally distinct steps of the recycling process along a complex and dynamic tubular network in which Rab11-FIPs occupy discrete domains.

Intelligent data analysis to model and understand live cell time-lapse sequences

BACKGROUND

One important aspect of cellular function, which is at the basis of tissue homeostasis, is the delivery of proteins to their correct destinations. Significant advances in live cell microscopy have allowed tracking of these pathways by following the dynamics of fluorescently labelled proteins in living cells.

OBJECTIVES

This paper explores intelligent data analysis techniques to model the dynamic behavior of proteins in living cells as well as to classify different experimental conditions.

METHODS

We use a combination of decision tree classification and hidden Markov models. In particular, we introduce a novel approach to "align" hidden Markov models so that hidden states from different models can be cross-compared.

RESULTS

Our models capture the dynamics of two experimental conditions accurately with a stable hidden state for control data and multiple (less stable) states for the experimental data recapitulating the behaviour of particle trajectories within live cell time-lapse data.

CONCLUSIONS

In addition to having successfully developed an automated framework for the classification of protein transport dynamics from live cell time-lapse data our model allows us to understand the dynamics of a complex trafficking pathway in living cells in culture.

A distinct trans-Golgi network subcompartment for sorting of synaptic and granule proteins in neurons and neuroendocrine cells

Golgi-to-plasma-membrane trafficking of synaptic-like microvesicle (SLMV) proteins, vesicular acetylcholine transporter (VAChT) and synaptophysin (SYN), and a large dense-core vesicle (LDCV) protein, chromogranin A (CgA), was investigated in undifferentiated neuroendocrine PC12 cells. Live cell imaging and 20°C block-release experiments showed that VAChT-GFP, SYN-GFP and CgA-RFP specifically and transiently cohabitated in a distinct sorting compartment during cold block and then separated into synaptic protein transport vesicles (SPTVs) and LDCVs, after release from temperature block. We found that in this trans-Golgi subcompartment there was colocalization of SPTV and LDCV proteins, most significantly with VAMP4 and Golgin97, and to some degree with TGN46, but not at all with TGN38. Moreover, some SNAP25 and VAMP2, two subunits of the exocytic machinery, were also recruited onto this compartment. Thus, in neuroendocrine cells, synaptic vesicle and LDCV proteins converge briefly in a distinct trans-Golgi network subcompartment before sorting into SPTVs and LDCVs, ultimately for delivery to the plasma membrane. This specialized sorting compartment from which SPTVs and LDCVs bud might facilitate the acquisition of common exocytic machinery needed on the membranes of these vesicles.

Cytokine release from innate immune cells: association with diverse membrane trafficking pathways

Cytokines released from innate immune cells play key roles in the regulation of the immune response. These intercellular messengers are the source of soluble regulatory signals that initiate and constrain inflammatory responses to pathogens and injury. Although numerous studies describe detailed signaling pathways induced by cytokines and their specific receptors, there is little information on the mechanisms that control the release of cytokines from different cell types. Indeed, the pathways, molecules, and mechanisms of cytokine release remain a "black box" in immunology. Here, we review research findings and new approaches that have begun to generate information on cytokine trafficking and release by innate immune cells in response to inflammatory or infectious stimuli. Surprisingly complex machinery, multiple organelles, and specialized membrane domains exist in these cells to ensure the selective, temporal, and often polarized release of cytokines in innate immunity.

Mechanisms and function of dendritic exocytosis

Dendritic exocytosis is required for a broad array of neuronal functions including retrograde signaling, neurotransmitter release, synaptic plasticity, and establishment of neuronal morphology. While the details of synaptic vesicle exocytosis from presynaptic terminals have been intensely studied for decades, the mechanisms of dendritic exocytosis are only now emerging. Here we review the molecules and mechanisms of dendritic exocytosis and discuss how exocytosis from dendrites influences neuronal function and circuit plasticity.

Imaging the Golgi apparatus in living mitotic cells

Live-cell imaging is a powerful tool which allows the observation of dynamic cellular processes while maintaining the native organization of the cell. Its advantages over other methods that disrupt cell integrity are abundantly evident in the study of cell division, where multiple subcellular organelles and molecules are involved in dynamic, spatio-temporally regulated processes such as Golgi and nuclear envelope disassembly/reassembly, spindle apparatus formation, chromosome condensation and segregation, and cytoplasmic division. This chapter will describe practical methods for cell synchronization, selection of fluorescent markers for transfection, and setting up imaging conditions and microscope parameters for acquiring time-lapse images of the Golgi apparatus in mitotic cells. These are general methods that can be applied to the study of many different types of organelles and molecules in dividing cells.

Spatial separation and bidirectional trafficking of proteins using a multi-functional reporter

Background

The ability to specifically label proteins within living cells can provide information about their dynamics and function. To study a membrane protein, we fused a multi-functional reporter protein, HaloTag^®, to the extracellular domain of a truncated integrin.

Results

Using the HaloTag technology, we could study the localization, trafficking and processing of an integrin-HaloTag fusion, which we showed had cellular dynamics consistent with native integrins. By labeling live cells with different fluorescent impermeable and permeable ligands, we showed spatial separation of plasma membrane and internal pools of the integrin-HaloTag fusion, and followed these protein pools over time to study bi-directional trafficking. In addition to combining the HaloTag reporter protein with different fluorophores, we also employed an affinity tag to achieve cell capture.

Conclusion

The HaloTag technology was used successfully to study expression, trafficking, spatial separation and real-time translocation of an integrin-HaloTag fusion, thereby demonstrating that this technology can be a powerful tool to investigate membrane protein biology in live cells.

Microtubule acetylation promotes kinesin-1 binding and transport

Long-distance intracellular delivery is driven by kinesin and dynein motor proteins that ferry cargoes along microtubule tracks . Current models postulate that directional trafficking is governed by known biophysical properties of these motors-kinesins generally move to the plus ends of microtubules in the cell periphery, whereas cytoplasmic dynein moves to the minus ends in the cell center. However, these models are insufficient to explain how polarized protein trafficking to subcellular domains is accomplished. We show that the kinesin-1 cargo protein JNK-interacting protein 1 (JIP1) is localized to only a subset of neurites in cultured neuronal cells. The mechanism of polarized trafficking appears to involve the preferential recognition of microtubules containing specific posttranslational modifications (PTMs) by the kinesin-1 motor domain. Using a genetic approach to eliminate specific PTMs, we show that the loss of a single modification, alpha-tubulin acetylation at Lys-40, influences the binding and motility of kinesin-1 in vitro. In addition, pharmacological treatments that increase microtubule acetylation cause a redirection of kinesin-1 transport of JIP1 to nearly all neurite tips in vivo. These results suggest that microtubule PTMs are important markers of distinct microtubule populations and that they act to control motor-protein trafficking.

The COPII cage: unifying principles of vesicle coat assembly

Communication between compartments of the exocytic and endocytic pathways in eukaryotic cells involves transport carriers - vesicles and tubules - that mediate the vectorial movement of cargo. Recent studies of transport-carrier formation in the early secretory pathway have provided new insights into the mechanisms of cargo selection by coat protein complex-II (COPII) adaptor proteins, the construction of cage-protein scaffolds and fission. These studies are beginning to produce a unifying molecular and structural model of coat function in the formation and fission of vesicles and tubules in endomembrane traffic.

Imaging Protein Trafficking

The need to understand complex intracellular trafficking mechanisms from both a basic and disease-oriented perspective has stimulated considerable interest in the development of real-time microscopy tools. Recent advances in instrumentation and the development of molecular bioprobes, such as green fluorescent protein and its derivatives, have opened up a new era in our ability to perform close to real-time imaging of cellular events with high spatial and temporal resolution, and with high sensitivity. This review briefly introduces and discusses some of the systems and methodologies that are available from several manufacturers, including laser scanning and spinning disk confocal microscopy, and total internal reflectance microscopy.

Specific serine proteases selectively damage KCNH2 (hERG1) potassium channels and I(Kr)

KCNH2 (hERG1) encodes the alpha-subunit proteins for the rapidly activating delayed rectifier K+ current (I(Kr)), a major K+ current for cardiac myocyte repolarization. In isolated myocytes I(Kr) frequently is small in amplitude or absent, yet KCNH2 channels and I(Kr) are targets for drug block or mutations to cause long QT syndrome. We hypothesized that KCNH2 channels and I(Kr) are uniquely sensitive to enzymatic damage. To test this hypothesis, we studied heterologously expressed K+, Na+, and L-type Ca2+ channels, and in ventricular myocytes I(Kr), slowly activating delayed rectifier K+ current (I(Ks)), and inward rectifier K+ current (I(K1)), by using electrophysiological and biochemical methods. 1) Specific exogenous serine proteases (protease XIV, XXIV, or proteinase K) selectively degraded KCNH2 current (I(KCNH2)) and its mature channel protein without damaging cell integrity and with minimal effects on the other channel currents; 2) immature KCNH2 channel protein remained intact; 3) smaller molecular mass KCNH2 degradation products appeared; 4) protease XXIV selectively abolished I(Kr); and 5) reculturing HEK-293 cells after protease exposure resulted in the gradual recovery of I(KCNH2) and its mature channel protein over several hours. Thus the channel protein for I(KCNH2) and I(Kr) is uniquely sensitive to proteolysis. Analysis of the degradation products suggests selective proteolysis within the S5-pore extracellular linker, which is structurally unique among Kv channels. These data provide 1) a new mechanism to account for low I(Kr) density in some isolated myocytes, 2) evidence that most complexly glycosylated KCNH2 channel protein is in the plasma membrane, and 3) new insight into the rate of biogenesis of KCNH2 channel protein within cells.